1. Field of the Invention
The present invention relates to a permanent magnet rotating electrical machine.
2. Discussion of the Background
Japanese Examined Utility Model Application Publication No. 7-36459, on page 9 and in FIG. 1, discloses a permanent magnet rotating electrical machine that includes a stator and a rotatable, approximately cylindrical rotor. The rotor includes a rotor core and the same number of permanent magnets as the number of magnetic poles. The rotor core constitutes pole shoes disposed in circumferential arrangement. Each of the permanent magnets is radially disposed between two of the pole shoes. In the rotor, magnetic fluxes are generated on the permanent magnets and directed to the circumference of the pole shoes, thus obtaining the magnetic poles.
FIG. 7 is a cross-sectional view of the rotor of the conventional permanent magnet rotating electrical machine. As shown in FIG. 7, the permanent magnet rotating electrical machine includes a stator and a rotatable, approximately cylindrical rotor.
When permanent magnets 3 on the circumferential sides of a pole shoe 2b generate magnetic fluxes at the respective magnetic flux generating surfaces of the permanent magnets 3, the magnetic fluxes are concentrated on the circumference of the pole shoe 2b, and this increases the gap magnetic flux density between the stator and the rotor, resulting in increased torque. A rotor core 2 includes a plurality of magnet accommodating holes 2d and pole shoes 2b integral with each other. When magnetic fluxes are generated on the magnetic flux generating surfaces of the permanent magnets, some of the magnetic fluxes become leakage magnetic fluxes returning to the permanent magnets in the rotor instead of reaching the circumference of the pole shoe 2b. It is important to reduce leakage magnetic fluxes for improving motor performance.
In an attempt to reduce leakage magnetic fluxes, the rotor core 2 of this conventional example includes a partially removed outer bridge 2e, a hole 4, and a reinforcing member 7. The partially removed outer bridge 2e is disposed over the outer end of each permanent magnet 3. The hole 4 is punched through an inner bridge 2a of the rotor core between two of the permanent magnets 3. The reinforcing member 7 is accommodated in the punched hole 4. The reinforcing member 7 is nonmagnetic and can be regarded as a void magnetically. When magnetic fluxes are generated on the permanent magnets 3, some of the magnetic fluxes become leakage magnetic fluxes returning to the permanent magnets 3 by way of joint portions 2c on the circumferential sides of each punched hole 4 and by way of the inner bridges 2a. The existence of the punched hole 4, however, keeps the leakage magnetic fluxes within restricted magnitude ranges.
Since the permanent magnets 3 are supported by the rotor core 2, the centrifugal force of the permanent magnets 3 is supported by the rotor core 2, while the toque of the permanent magnets 3 and the rotor core 2 is supported by a shaft 9 via the rotor core 2.
Another permanent magnet rotating electrical machine is disclosed in Japanese Patent Publication No. 3224890, on page 9 and in FIG. 1. In the rotor of the permanent magnet rotating electrical machine, the toque of permanent magnets and rotor cores on their circumferences is supported by the shaft through side plates.
FIG. 8A is a side view of the rotor of the other conventional permanent magnet rotating electrical machine, and FIG. 8B is a cross-sectional view of the rotor. As shown in FIGS. 8A and 8B, the permanent magnet rotating electrical machine includes a stator and the rotor 10, which is rotatable and has an approximately cylindrical shape.
As shown in FIGS. 8A and 8B, the rotor 10 includes rotor cores 16 and the same number of permanent magnets 14 as the number of magnetic poles. The rotor cores 16 are separated from each other on a magnetic pole basis and constitute pole shoes disposed in circumferential arrangement. Each of the permanent magnets 14 is radially disposed between two of the pole shoes. The rotor cores 16 and the permanent magnets 14 are secured between side plates 24. The rotor cores 16 are separated into independent rotor cores each corresponding to one pole, and secured to the side plates 24 through respective rods 22. The side plates 24 are secured to a shaft 12.
The rotor cores 16 are separated from each other on a magnetic pole basis by the permanent magnets 14, and this allegedly reduces leakage magnetic fluxes shortcutting from the N pole to the S pole. Accordingly, the magnetic fluxes generated on the permanent magnets 14 are for the most part directed to the gap between the stator and the rotor 10, resulting in an increased possible maximum torque compared with permanent magnet rotating electrical machines with bridges.